Vapor temperature control method



Aug. 24, 1965 w. w. SCHROEDTER 3,202,138

VAPOR TEMPERATURE CONTROL METHOD Filed July 27. 1961 WITHOUT RECJE'CULATION WITH mzcnzcuumon Q G :3 a g 5 P WORK\NC| FLUID TEMPERATURE FIG. I

INVENTOR: WILLBURT W. SCHROEDTER PM 4 12w ATTORNEY United States Patent 3,292,138 VAPOR TEMPERATURE CONTROL METHOD Willburt W. Schroedter, West Hartford, Conn, assignor to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed July 27, 1961, Ser. No. 127,176 3 Claims. (Cl. 122-4179) This invention relates generally to forced through-flow vapor generators operating at supercritical pressures and on the reheat cycle and has particular relation to an improved method of vapor temperature control for such umts.

The invention is characterized by a forced through-flow vapor generator operating at supercritical pressures and including an elongated furnace which has heat absorbing tubes disposed along its walls which form part of the through-flow system with combustion gases passing longitudinally of the furnace and through the furnace outlet with these gases passing over convection heat absorbing surface wherein the working medium which is supplied to a turbine is reheated to a predetermined desired temperature after a portion of its energy has been utilized in the turbine.

In accordance with the invention the heat absorption within the furnace is varied by means of varying the zone of combustion in the furnace and simultaneously recirculating fluid through the furnace wall tubes so as to vary the heat content of the gases passing over the convection heat absorbing surface in a manner which will tend to, at least in part, offset the effect of variation of load on the reheat vapor temperature.

The supercritical vapor generator with which the invention is concerned has the walls of the furnace lined with fluid heating tubes which form part of the through-flow circuit of the generator. The arrangement is such that a header is provided in the lower region or lower end of the furnace with which tubes which extend upwardly therefrom are connected. These tubes effectively line the furnace walls. Superimposed upon the through-flow system is a circulating system which is effective to circulate fluid through the furnace wall tubes or at least the furnace Wall tubes that line the lower region of the furnace. This recirculation of the working medium through the furnace wall tubes is effective in such a manner that below a predetermined load on the unit the recirculation is initiated and as the load decreases below this predetermined load the proportion of the recirculated fluid to the total flow through the tubes is progressively increased. The effect of this recirculation of the fluid through the furnace wall tubes is to increase the temperature of the working medium entering these tubes and decrease the difference in temperature of the Working medium between the inlet and outlet of the tubes with there being an increase in the average temperature of the medium in the tubes. This in turn will decrease the convection and radiant heat pickup of the furnace wall tubes or more correctly the working medium upon traversal of these tubes for a given firing rate because the heat head, i.e. the difference in temperature between the heated and the heating medium, is decreased. Simultaneously with this recirculation of working medium through the furnace wall tubes and which recirculation is increased with decreasing load the zone of combustion is varied so that as the load on the unit is decreased recirculation of the fluid medium is increased and the firing zone is moved in the direction toward the furnace outlet. This moving of the firing zone has the effect of decreasing the heat absorption of the fluid traversing the furnace wall tubes, this supplementing the same effect that is provided by recirculation of the fluid medium as previously explained. Accordingly the effect of these operations is to decrease the heat absorption in ice the furnace and make more heat available to the convection reheat surface for a given firing rate. Thus by moving the zone of combustion toward the furnace outlet and increasing the recirculation of fluid medium through the furnace wall tubes with decrease in load the tendency of the temperature of the reheat vapor to decrease is over come at least in part.

It is noted that by employing recirculation of the working medium together with adjusting the zone of combustion, the operating characteristic within the through flow circuit, and particularly the furnace walls, is improved with there being greater stability and with there being more uniform temperature of the tubes in addition to effecting the desired and relatively substantial control of the reheat vapor temperature. Varying the combustion zone may have a tendency to cause substantial differences in temperatures of the tubes throughout their length with the recirculation of the working medium offsetting and compensating for this tendency.

Accordingly it is an object of the present invention to provide an improved control arrangement and scheme for forced through-flow vapor generators operating at supercritical pressure on the reheat cycle and wherein the reheat is effected predominately by convection heat exchange.

Another object of the invention is to provide such an improved control wherein the heat absorption in the furnace is varied with varying load both by means of adjusting the zone of combustion with relation to the furnace outlet and by means of recirculating fluid through the tubular walls lining the furnace.

The invention will be more fully understood from the following description when considered in conjunction with the accompanying drawings forming a part thereof and in which:

FIG. 1 is a vertical section through a high capacity vapor generator operating at supercritical pressure and organized for carrying out the improved method of the invention with this illustration being somewhat diagrammatical in nature;

FIG. 2 is a transverse sectional view through the furnace of FIG. 1 being taken generally along line 2--2 of FIG. 1;

FIG. 3 is a graphic representation illustrating the fluid temperature rise in the furnace walls at a particular load on the generator organization.

Referring now to the drawings, wherein like reference characters are used throughout to designate like elements, the illustrative organization depicted therein includes a supercritical vapor generator having a furnace which is generally designated 10 and upwardly through which combustion gases pass with the furnace outlet being located at the upper region of the furnace and with lateral gas pass 12 extending from this outlet and being connected to the upper end of gas pass 14. This latter gas pass extends downwardly in generally parallel relation with the furnace and connects, through duct 16, with a suitable air heater or the like which in turn is connected to a stack.

The through-flow circuit of the supercritical generator includes economizer 18 positioned in the lower region of gas pass 14. This economizer, which is preferably comprised of sinuously bent tubes, receives the through-flow at supercritical pressure from feed pump 20 and after traversing the economizer the working :medium passes upwardly to the distribution header 22. From this distribution header the working medium is conveyed through conduit 24 to mixing vessel 26 and then from this mixing vessel through conduit 28 to the distribution header 30. From this header the fluid is distributed to the furnace wall tubes 32 which extend upwardly along all four of the furnace walls, lining the inner surface thereof, with the tubes being in side-by-side relation and in parallel flow relation with respect to the working medium so the upflow of the fluid medium through these tubes is provided. As illustratively disclosed in FIG. 1 the tubes 32 which line the lower region of the furnace extend from header 30 to sub-headers 34 and then continue on from these sub-headers up the remaining vertical extent of the furnace walls with these tubes eventually terminating in header 36. From this header 36 the through flow is conveyed through conduits 38 to the header 40 and from this header 4th the working medium flows through the tube bundle or heat exchange section 42 being discharged from this section into header 44. From header 44 the working fluid is conveyed through conduit 46 to header 48 and from this header through the heat exchange tubular panels 50 with the vapor from these panels being collected in header 52 and then conveyed through conduit 54 to the turbine 56.

Disposed within the lateral gas pass 12 is the reheater 58 which is predominately convection heating surface and is comprised of a series of rows of tubes disposed in side-by-side relation across the gas pass with this reheater receiving vapor from the turbine through the conduit 60 and with the reheater being effective to reheat the vapor to the desired temperature, with this vapor then being reintroduced into the turbine via conduit 62.

The discharge from this turbine is received by condenser 64 which is effective to condense the Vapor and the condensate is pumped by condensate pump 66 through the feed water header 68, deaerator 7% and then to the feed pump 20.

Since the vapor generator is operated at supercritical pressure, as for example 3500 lbs. per sq. in. delivered to the turbine, the working medium will continuously rise in temperature as it traverses the through-flow circuit. Accordingly as the working fluid passes from header 30 up through the wall tubes 32 it progressively increases in temperature. For example, at a given load such as 30 percent load the temperature of the medium entering header 30 may be 540 while that discharged to header 36 may be 750.

It is a characteristic of vapor generators to which the invention is directed that the reheat vapor delivered to the turbine has a tendency to fall in temperature as the load on the generator is decreased. Since it is necessary that this reheat temperature delivered to the turbine be maintained constant throughout the operating load range of the unit, corrective measures must be taken to overcome this tendency and to control this temperature in a manner so that it may be retained at its desired value notwithstanding variation in load. In accordance with the present invention this tendency of the reheat vapor temperature to decrease in load is overcome at least in part by means of adjusting the zone of combustion in the furnace and by means of simultaneously recirculating fluid medium through the furnace wall tubes. The combination of these actions provides a corrective effect that has a substantial influence on the vapor temperature over a relatively wide load range.

The recirculation of the working medium through the furnace wall tubes or at least through the furnace wall tubes lining the lower region of the furnace is effective to decrease the heat absorption in the furnace. This is accomplished by providing a recirculating system superimposed on the through-flow system and which is connected with the through-flow system at a point downstream of the furnace walls relative to flow of the working medium and to a point upstream of the furnace walls, with a suitable pump means being provided to effect recirculation through this system. In the illustrative organization of FIG. 1 there is connected with the outlet header 36, which is the collecting header for the parallel tubes lining the four furnace walls, conduit 82 which extends to and is connected with pump means 84. Connected with the outlet of this pump means is conduit 86 which in turn is connected with the mixing vessel 26, with check valve 88 being provided in this conduit in order to prevent reverse flow from the mixing vessel to and through the pump 84. Mixing vessel 26 is effective to mix the hot fluid pumped by pump 84 from header 36 with the relatively cold through-flow coming from economizer 18. It will be seen that this recirculating system is so arranged that a portion of the working medium may be withdrawn from the header 36 and returned via conduit 82, pump 84, conduit 86, mixing vessel 26 and conduit 28 to the inlet header 3! for repassage up through the parallel tubes 32 lining the furnace walls.

The effect of recirculating a portion of the working medium through the parallel furnace wall tubes 32 is to increase the average temperature of the medium in these tubes with there being a substantial increase in the temperature of the medium at the inlet or the lower end of the tubes over that which would prevail without recirculation, and with the temperature rise of the medium, due to recirculation, becoming progressively smaller as the flow progresses up through the tubes. This effect is illustrated in FIG. 3 which is a graphic representation at one load of the unit of the temperature of the fluid medium passing through the furnace wall tubes 32 both with and without recirculation. The dotted line curve 90 represents the temperature of the working fluid passing up through the furnace wall tubes without recirculation, while the solid line curve 92 represent this temperature with recirculation with both of these curves indicating this temperature at a particular load, such as 30 percent load, since the temperature will change somewhat as the load changes. From these FIG. 3 curves it will be seen that by means of recirculating a portion of the working medium through the furnace Wall tubes the temperature of the fluid entering these tubes may be raised substan tially and of course the average temperature throughout the length of the tubes will be raised with the greatest effective rise in temperature being provided in the lower region of the furnace. This is the region where the heat in the furnace is most intense and by means of this recirculation the convective and radiant heat transfer to the medium flowing through the tubes may be varied since the heat head or temperature differential between the heating and the heated medium will be varied by this recirculation. By raising the temperature of the working medium passing through the tubes and particularly at the lower region of the tubes 32 the temperature difference between the heating and the heated medium will be decreased so that the amount of heat transferred to the fluid flowing through tubes 32 will be decreased resulting in the gases passing over the superheat and reheat heat exchange surfaces having a greater heat content.

As previously mentioned the vapor generator with which the invention is concerned has the characteristic of having the reheat vapor temperature tend to fall with decreasing load on the unit. This tendency is overcome at least in part in accordance with the present invention by means of simultaneously adjusting the zone of combustion in the furnace and recirculating fluid medium through the furnace Wall tubes as previously described. The control arrangement is such that as the load on the unit decreases, for instance, below a predetermined load such as 90 percent of full load, the zone of combustion is adjusted toward the furnace outlet and a portion of the working medium is recirculated through the furnace wall tubes. This results for a given firing rate in decreasing the heat absorption in the furnace and making a greater amount of heat available in the combustion gases passing over the heat exchange surface disposed in the combustion gas stream and accordingly imparting more heat to the fluid flowing through these surfaces thereby raising the temperature of the reheat vapor over what it would otherwise be.

While the illustrative organization provides for recirculation throughout the entire furnace height it may be advantageous to recirculate the working medium only through the lower furnace region, with this being the region of most intense heat as previously mentioned. This may be achieved by connecting the inlet of the circulating system with headers 34 or some other suitable header arrangement may be provided. Furthermore it is not necessary to arrange furnace wall tubes 32, so that the fluid medium flows in parallel flow relation only in an upper direction through these tubes. The benefit of reduced heat absorption by recirculation of the working medium may be provided by having tubes 32 extend up and down one or more times along the furnace wall or along the lower portion of the furnace wall. However, the optimum effect is obtained with the parallel flow arrangement wherein each of the side-by-side tubes 32 is a single upwardly extending tube connected between the inlet and outlet headers.

Burners 82 are mounted in the furnace as indicated in the drawings so as to project fuel and air tangentially of a generally centrally located upright cylinder. These burners are adjustable so that the zone of firing may be moved vertically of the furnace between a position designated generally A and a position designated generally B. The burners may be adjusted so as to locate the zone of combustion at or anywhere between the extreme locations of A and B.

In lieu of moving the zone of combustion by adjusting burners such as disclosed in FIGS. 1 and 2 the zone of combustion may be varied longitudinally of the furnace by mounting several burners or several sets of burners in vertically spaced positions in the furnace wall and controlling these burners so that a desired set or a desired number of sets are in operation which corresponds with the vertical extent of the furnace where it is desired to have the zone of combustion. In either system, i.e. whether fixed vertically spaced burners are employed or tilting tangential burners are employed, the control effect that is produced is to vertically adjust or control the location of the zone of combustion in the furnace. Adjusting the zone of combustion from the lower portion or location B toward the upper location A, while maintaining the firing rate constant, increases the heat content of the gases passing over the reheater 58 since the length of the furnace and accordingly the amount of heat absorption therein is elfectively decreased. This, together with increasing the temperature of the fluid flowing through the furnace wall tubes by recirculation of the working fluid, as described previously and indicated in FIG. 3 substantially increases the heat content of the gases passing over the reheater 58 over what it would otherwise be with decreasing load. Accordingly, with the invention the tendency of the reheat temperature to fall with decrease in load is overcome at least in part by simultaneously recirculating the supercritical pressure working fluid through the furnace wall tubes 32, which make up a portion of the through-flow system, and adjusting the zone of combustion in the furnace from its lower location towards its upper location. As the load is progressively decreased the amount of fluid recirculated is increased and the location of the combustion zone is progressively moved further toward the upper location A.

This simultaneous recirculation of working fluid through tubes 32 and adjustment of the combustion zone in the furnace causes the heat content in the gases passing over the predominately convective reheater 58 to increase with decrease in load over that which would prevail without this control action. Accordingly the tendency of the reheat temperature to fall with a decrease in load is offset at least in part.

In operation of units of the type to which the invention pertains, i.e. supercritical units which are provided with through-flow circuits, the temperature of the primary vapor is controlled by the firing of the unit and the pressure is controlled by the feed or through-flow pump. A spray attemperator 94 may be incorporated intermediate the heat exchange portion 42 and the final or finishing heat exchange portion 50 of the unit in order to provide more sensitive and accurate control and to prevent overrunning of the primary vapor temperature. However the principal control for this primary vapor temperature is achieved by controlling the firing of the unit with the firing being decreased to decrease the primary vapor temperature and increased to increase the same. The reheat vapor temperature is controlled through the control actions previously described. The reheat vapor temperature delivered to the turbine may be sensed, such as at the location 85, and the indication thus achieved may be employed through known control organizations to obtain adjustment or regulation of the control actions previously described so as to maintain this temperature of the reheat vapor generally constant throughout a substantial load range.

With the novel process of the invention a substantial control action or effect with relation to reheat control is obtained and at the same time operating conditions within the furnace wall tubes, such as stability and temperature balance, will be improved over what they would otherwise be.

Accordingly it will be appreciated that there is provided in accordance with the present invention an improved method where, in a supercritical, forced throughflow vapor generator, the tendency of the reheat vapor temperature to rise with decreasing load is overcome at least in part by means of an improved method of operating such an organization.

It will be understood that the foregoing description is intended for the purpose of illustration only and that modifications such as will occur to those skilled in the art are possible and are embraced within the scope and spirit of the invention.

What I claim is:

1. In a forced through-flow vapor generator operating at supercritical pressure and on the reheat cycle and which generator includes an elongated furnace through which combustion gases are conveyed with the furnace having a combustion gas outlet and fired at a location remote from said outlet and with the walls of the furnace being lined with tubes which form part of the supercritical through-flow system, said generator also including predominantly convection reheating surface over which said combustion gases are conveyed, said generator having the characteristic that with decrease in load the reheat temperature tends to decrease the method of increasing the heat input to said vapor reheating surface over what it would otherwise be with decreasing load thereby overcoming, at least in part, the tendency of the reheat temperature to decrease with decrease in load, comprising recirculating a portion of the working medium through at least a portion of the through-flow system lining the furnace Walls by withdrawing working medium from the through-flow system at a location downstream of the portion lining the furnace walls and introducing it into said system upstream of said portion with generally the entire heat content of this withdrawn fluid being contained in the fluid entering the portion of the circuit lining the furnace walls and simultaneously adjusting the zone of combustion longitudinally within the furnace toward said outlet, increasing the proportion of the fluid circulated relative to the through-flow and moving the combustion zone toward said outlet as the load on the vapor generator decreased.

2. The method of generating vapor at supercritical pressure and reheating the same after a portion of its energy has been dissipated comprising generating a combustion gas stream by the burning of fuel in a combustion zone, forcing the working medium at supercritical pressure through a continuous path and imparting heat generated by the burning fuel to said medium during its traversal of said path and in sufiicient quantity to heat said medium to a desired value, during its conveyance along said path passing the working medium in confined streams in bounding relation with said combustion zone and with said combustion gas stream and imparting heat thereto from said zone and stream to said bounding streams, utilizing a portion of the energy of the working medium heated to said desired temperature and thereafter reheating the same by reconveying said medium in heat exchange relation with the combustion gas stream and imparting heat therefrom principally by convection and at a zone downstream of said combustion zone with the characteristic prevailing that incident to decrease in vapor generating rate the temperature of this reheated vapor tends to decrease, overcoming this characteristic at least in part by recirculating in said confined streams working medium heated by being passed in bounding relation with said combustion zone with this recirculated working medium being withdrawn from said path at a location downstream of the location Where it is passed in confined streams in bounding relation with said cornbustion zone and reintroduced into said path upstream of such location and in a condition such that generally the entire heat content of this withdrawn working medium is contained in said confined streams that are in bounding relation with said combustion zone and simultaneously adjusting said combustion zone in the direction of the combustion gas flow toward said convection zone, increasing the recirculation rate and moving said combustion zone further toward said convection zone as the load on the vapor generator decreases.

3. In a vapor generator producing superheated vapor at supercritical pressure, operating on the reheat cycle, and including a forced through-flow system through which the working medium is conveyed, said generator having a vertically disposed furnace the walls of which are lined with side-by-side longitudinally extending tubes upwardly through which the working medium is directed in parallel flow relation with the furnace being fired adjacent its lower 3 end and so combustion gases pass upwardly therethrough, there being an offtake in the upper furnace portion through which the gas is passed, and with the generator including reheating surface absorbing heat from said gases primarily by convection; the method of overcoming at least in part the tendency of reheat vapor temperature to fall with decreasing load characterized by adjusting the combustion zone longitudinally of the furnace and toward the 5 furnace outlet and recirculating a portion of the working medium which has traversed the furnace wall tubes by withdrawing working medium from the through-flow system at a location downstream of these furnace wall tubes and introducing it into said system upstream of these 10 furnace wall tubes and in a condition such that generally the entire heat content of this withdrawn working medium is contained in the working medium entering these furnace Wall tubes back through these wall tubes, moving the combustion gas zone nearer to said outlet 15 and increasing the recirculation of the Working medium with decrease in load.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS 4/58 Great Britain. 3/60 Great Britain.

OTHER REFERENCES German printed application No. 1,056,147, 4/ 59. Combustion, of August 1956, pages 47 to 56, published 5 by Combustion Publishing Co., Inc., of New York.

PERCY L. PATRICK, Primary Examiner.

FREDERICK L. MATTESON, JR., KENNETH W.

SPRAGUE, Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,202,138 August 24, 1965 Willburt W. Schroedter It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 8, line 13, strike out "back through these wall tubes," and insert the same after "tubes", in line 6, same column 8.

Signed and sealed this 23rd day of August 1966.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNEI Commissioner of Patents 

1. IN A FORCED THROUGH-FLOW VAPOR GENERATOR OPERATING AT SUPERCRITICAL PRESSURE AND ON THE REHEAT CYCLE AND WHICH GENERATOR INCLUDES AN ELONGATED FURNACE THROUGH WHICH COMBUSTION GASES ARE CONVEYED WITH THE FURNACE HAVING A COMBUSTION GAS OUTLET AND FIRE AT A LOCATION REMOTE FROM SAID OUTLET AND WITH THE WALLS OF THE FURNACE BEING LINED WITH TUBES WHICH FORM PART OF THE SUPERCRITICAL THROUGH-FLOW SYSTEM, SAID GENERATOR ALSO INCLUDING PREDOMINANTLY CONVECTION REHEATING SURFACE OVER WHICH SAID COMBUSTION GASES ARE CONVEYED, SAID GENERATOR HAVING THE CHARACTERISTIC THAT WITH DECREASE IN LOAD THE REHEAT TEMPERATURE TENDS TO DECREASE THE METHOD OF INCREASING THE HEAT INPUT TO SAID VAPOR REHEATING SURFACE OVER WHAT IT WOULD OTHERWISE BE AT DECREASING LOAD THEREBY OVERCOMING, AT LEAST IN PART, THE TENDENCY OF THE REHEAT TEMPERATURE TO DECREASE WITH DECREASE IN LOAD, COMPRISING RECICULATING A PORTIN OF THE WORKING MEDIUM THROUGH AT LEAST A PORTION OF THE THROUGH-FLOW SYSTEM LINING THE FURNACE WALLS BY WITHDRAWING WORKING MEDIUM FROM THE THROUGH-FLOW SYSTEM AT A LOCATION DOWNSTREAM OF THE PORTION LINING THE FURNACE WALLS AND INTRODUCING IT INTO SAID SYSTEM UPSTREAM OF SAID PORTION WITH GENERALLY THE ENTIRE HEAT CONTENT OF THIS WITHDRAWN FLUID BEING CONTAINED IN THE FLUID ENTERING THE PORTION OF THE CIRCUIT LINING THE FURNACE WALLS AND SIMULTANEOUSLY ADJUSTING THE ZONE OF COMBUSTION LONGITUDINALLY WITHIN THE FURNACE TOWARD SAID OUTLET, INCREASING THE PORTIN OF THE FLUID CIRCULATED RELATIVE TO THE THROUGH-FLOW AND MOVING THE COMBUSTION ZONE TOWARD SAID OUTLET AS THE LOAD ON THE VAPOR GENERATOR DECREASED. 